Most rear wheel drive vehicles which are selectively engagable to be all wheel drive vehicles have a transfer case. The transfer case typically has a primary shaft which is connected to the output of a vehicle transmission. The primary shaft typically rotatably mounts a sprocket that engages a chain. The chain typically engages with another sprocket that is typically connected with a secondary shaft. The secondary shaft is rotatably mounted within the transfer case. The secondary shaft is torsionally connected with a front drive shaft that is torsionally connected with a front differential to power front wheels of the vehicle. To provide a function of selectively engaging the front wheels of the vehicle, there is provided a clutch mechanism that selectively engages the sprocket on the primary shaft with the primary shaft to transfer torque from the primary shaft to the secondary shaft thereby selectively engaging the front wheels of the vehicle. When ideal pavement conditions exist, typically the clutch will not be engaged so that the transmission only torsionally engages the rear wheels of the vehicle. A controller is typically provided which is cognizant of wheel slip conditions to automatically control the engagement of the clutch. In some four wheel drive embodiments, the clutch can also be engaged as a result of operator demand.
In many transfer cases, in the two wheel drive mode of operation, the secondary shaft will still rotate due to the movement of the non-powered (in most cases front) wheels. This condition exists even when the clutch is non-engaged and no torque transfer is occurring to the secondary shaft.
Most transfer cases have some type of lubricant pump to provide lubrication for the clutch in its associated friction pack, the shafts, bearings, sprockets and chain. The operation of this pump provides a load on the vehicle engine either mechanically by being powered by the take off from the vehicle drive train or is a load upon the engine indirectly by generating a parasitic electrical load. It is desirable to provide a transfer case wherein the lubrication needs can be provided in a more passive manner from the operation of the transfer case.
It is desirable that the amount of lubricant within the transfer case which is exposed to churning by the secondary sprocket be as little as possible to avoid foaming the lubricant and the generation of heat within the lubricant. The above provides higher efficiency of the transfer case when churning of the oil is reduced or minimized. It is desirable to provide a transfer case which maximizes lubricant capacity, minimizes the distance from the sprocket wheel to the bottom of the transfer case and additionally minimizes the amount of fluid exposed to the secondary sprocket during two wheel drive operation or high speed four wheel drive operation wherein there is low torque demands for the wheels connected to the differential connected with the secondary shaft.